LTC1867LIGN datasheet(11/16 Pages) LINER

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LTC1863L/LTC1867L

1863L7Lf

used since these components can add distortion. NPO and

silver mica type dielectric capacitors have excellent linear-

ity. Carbon surface mount resistors can also generate

distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resis-

tors are much less susceptible to both problems.

DC Performance

One way of measuring the transition noise associated with

a high resolution ADC is to use a technique where a DC

signal is applied to the input of the ADC and the resulting

output codes are collected over a large number of conver-

sions. For example, in Figure 2 the distribution of output

codes is shown for a DC input that had been digitized 4096

times. The distribution is Gaussian and the RMS code

transition noise is about 1.6LSB.

APPLICATIO S I FOR ATIO

Figure 2. LTC1867L Histogram for 4096 Conversions

Figure 3a. LTC1867L Nonaveraged 4096 Point

FFT Plot with 2.7V Supply

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 3a shows a typical SINAD of 81.4dB with

a 175kHz sampling rate and a 1kHz input. Higher SINAD can

be obtained with a 3V supply. For example, when an exter-

nal 3V is applied to REFCOMP (tie VREF to GND), a SINAD

of 83.5dB can be achieved as shown in Figure 3b.

Figure 3b. LTC1867L Nonaveraged 4096 Point

FFT Plot with 3V Supply

CODE

200

400

600

800

1200

1863L7L G12

1000

465

1044

895

830

261

23 9 1

170

333

VDD = 2.7V

INTERNAL REF

Dynamic Performance

FFT (Fast Fourier Transform) test techniques are used to

test the ADC’s frequency response, distortion and noise at

the rated throughput. By applying a low distortion sine

wave and analyzing the digital output using an FFT algo-

rithm, the ADC’s spectral content can be examined for

frequencies outside the fundamental.

Signal-to-Noise Ratio

The Signal-to-Noise and Distortion Ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

FREQUENCY (kHz)

–60

–40

65.625

1863L7L G03

–80

–100

21.875

43.75

87.5

–120

–140

–20

fSAMPLE = 175ksps

fIN = 1kHz

SNR = 82.9dB

SINAD = 81.4dB

THD = 86.8dB

FREQUENCY (kHz)

–60

–40

65.625

1863L7L F03b

–80

–100

21.875

43.75

87.5

–120

–140

–20

fSAMPLE = 175ksps

fIN = 1kHz

SNR = 84.7dB

SINAD = 83.5dB

THD = 90dB

REFCOMP = EXT 3V

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

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